DAMASCUS - At least 50 Islamic State (IS) militants were killed Thursday by US-led airstrikes on a caves area in eastern Syria, a war monitor reported.

The air raids targeted the caves in the Baghouz, the last IS redoubt in the eastern countryside of Deir al-Zour province in eastern Syria, according to the Syrian Observatory for Human Rights.

The UK-based watchdog group said the IS militants were hiding in caves and underground bunkers after they had lost control over Baghouz.

It said the US-backed Syrian Democratic Forces (SDF) is still combing the Baghouz area in search for IS militants.

On March 23, the SDF declared the defeat of IS in eastern Syria after launching a campaign last September to eliminate the extremist organization's gunmen on the eastern bank of Euphrates River.